Pulse width modulated control system with external feedback and mechanical memory

ABSTRACT

This invention comprises a control system for electrically controlled, mechanically actuated mechanisms of large power rating. Such mechanisms normally include mechanical actuating devices such as control valves for controlling the output operating condition of the mechanism. The control system of the invention includes electric power modulation circuit means having its input coupled to a source of electric control signals for controlling the operation of the mechanism and for producing output modulated controlled electric power signals. These output modulated controlled power signals are applied to an electromechanical positioning device such as a servomotor which has its output mechanically coupled to and controlling the position of the mechanical actuating means (valve) of the mechanism. The input of the servomotor is coupled to and controlled by the output from the electric power modulation circuit. The electric power modulation circuit preferably comprises a pulse width modulation circuit having a source of constant period pulsed clock signals supplied thereto and two output terminals. The input control signal selectively controls the width at the clock rate of the output signal pulses produced at the output of the modulation circuit as well as selects the output terminal to which the pulse width modulated pulses are supplied. As a result, the pulse width modulated output signals from one output terminal will serve to adjust the servomotor in one direction while the signals from the remaining output terminal serve to adjust the servomotor in the other direction. By reason of this arrangement, the last adjusted position of the mechanical actuating means by the servomotor serves as a memory for the overall control system until the next adjustment of the operating condition by the control system. The control system of the invention is particularly intended for use with turbinegenerator sets and for controlling the startup and initial loading of the turbine-generator set until such time that it has attained its optimum operating condition.

United States Patent [72] Inventor William 1).Cockrell Waynesboro, Va.[21] AppLNo. 681,819 [22] Filed Nov. 9, 1967 [45] Patented Feb. 16, 1971[73] Assignee General Electric Company a corporation of New York [54]PULSE WIDTH MODULATED CONTROL SYSTEM WITH EXTERNAL FEEDBACK ANID 254,20.050, 20.290, 20.300, 20.310; 3 1 8/20.085, 341, (lnquired); 244/77,77D, (Inquired); 415/1, 17, (Inquired); 290/40, (Inquired) [56]References Cited UNITED STATES PATENTS 3,184,188 5/1965 Rossire3l8/(20.085) 3,446,224 5/1969 Zwicky... 415/17 3,486,100 12/1969 James318/341 3,105,927 10/1963 Flatten etal 3l8/(20.310) 3,131,340 4/1964Johnson et a1. ....3 l 8/28(20.290) 3,181,046 4/1965 Sutton.....3l8/28(20.290) 3,238,376 3/1966 Ernst et a1 290/40XL 3,244,8984/1966 Hickox 290/40XR 3,274,443 9/1966 Eggenberger et al. 290/40XV3.4l6,052 12/1968 Russell et al ..3 18/1 8(20290) POWER TO RAISECLOCKIPULSE TIMING) PROTECTIVE SIGNALS FEED BACK SIGNAL TRANSDUCERS ANDAMPLIFIERS TEMPERATURE VIBRATION SPEED EXPANSION ECCENTRICITY ELONGATION Primary Examiner-Q. R. Simmons p Attorneys- Lawrence G. Norris,Michael Masnik, Stanley C.

Corwin, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. GoldenbergABSTRACT: This invention comprises a control system for electricallycontrolled, mechanically actuated mechanisms of large power rating. Suchmechanisms normally include mechanical actuating devices such as controlvalves for controlling the output operating condition of the mechanism.The control system of the invention includes electric power modulationcircuit means having its input coupled to a source of electric controlsignals for controlling the operation of the mechanism and for producingoutput modulated controlled electric power signals. These outputmodulated controlled power signals are applied to an electromechanicalpositioning device such as a servomotor which has its outputmechanically coupled to and controlling the position of the mechanicalactuating means (valve) of the mechanism. The input of the servomotor iscoupled to and controlled by the output from the electric powermodulation circuit. The electric power modulation circuit preferablycomprises a pulse width modulation circuit having a source of constantperiod pulsed clock signals 7 supplied thereto and two output terminals.The input control signal selectively controls the width at the clockrate of the output signal pulses produced at the output of themodulation circuit as well as selects the output terminal to which thepulse width modulated pulses are supplied. As a result, the pulse widthmodulated output signals from one output terminal will serve to adjustthe servomotor in one direction while the signals from the remainingoutput terminal serve to adjust the servomotor in the other direction.By reason of this arrangement, the last adjusted position of themechanical actuating means by the servomotor serves as a memory for theoverall control system until the next adjustment of the operatingcondition by the control system. The control system of the invention isparticularly intended for use with turbine-generator sets and forcontrolling the startup and initial loading of the turbine-generator setuntil such time that it has attained its optimum operating condition.

LOAD FEEDBACK l8 MEASURED QUANITIES 0R LOWER EHC REFERENCE 27 MAINCOMPUTATION SECTION A L PROPORTIONAL) EN ET l PRODUCES ANALOG MONITORS:

LOWEST VALUE/OF SIGNALS REPRESENTING: VIBRATION SIGNAL A EN E STRESSMARGINS FOR PREDICTED VIB.

SIGAqLSLG BORE AND SHELL DIFF. EXPANSION S PREDICTED STRESS ROTORELONGATION GVSTAC MARGINS ECCENTRICITY EQUIPMENT 28 OPERATOR ALARMSPATENTEDFEMDIQYI 3564.273

3 saw 2 BF 4 FIG 2 DIODE GATESY FROM DA BOARDS INVENTOR. WILLIAM D.COCKRELL HIS ATTORNEY I 29 I v I v FROM 57 FIG.2

' PATENTEU mu 6197i 3.564273 sum 3 BF 4 FIG. 3

INVENTOR.

HIS ATTORNEY PATENIED FEBI 6 ISYI I 3564-273 SHEET t [IF 4 TO JUNCTIONOF 84 8 85 FIG.3 1

TD BASE To INVENTOR. WILLIAM D. COCKRELL HIS ATTORNEY PULSE WIDTHMODULATED CONTROL SYSTEM WITH EXTERNAL FEEDBACK AND MECHANICAL MEMORYBACKGROUND OF INVENTION 1. Field of invention This invention relates toa new and improved control system for electrically controlled,mechanically actuated mechanisms of large power rating such asturbine-generator sets.

The modern steam turbine which generates a half million horsepower ofelectricity is a rather sophisticated machine. The boiler generates thesteam which flows past blades mounted in the shell which deflect it ontobuckets attached to the revolving rotor. After expending part of itsenergy pushing on the first row of buckets, it is again deflected by asecond row of blades onto another row of buckets and so on through manystages until its energy-is spent and it is collected in the exhaust hoodto be finally chilled and condensed back to water and pumped again intothe boiler.

To obtain maximum efficiency very high temperatures and pressures areused. This requires very special steel alloys to withstand the highforces and remain strong at the elevated temperatures. Also these largeunitsmust have very thick rotor shafts. The heat of the steam flowingover the rotor penetrates the metal at a rather slow rate so that as thehotter outer material tries to expand, severe thermal stresses can beset up in the rotor material.

To make matters more difficult, maximum efficiency demands'that afterpassing through a number of stages, the steam be returned to the boilerfor the addition of more heat energy before returning to the lowerpressure stages. At times this is an entirely separate unit.

There is one point of control of the steam flow. This is the entrance tothe first high pressure stage. Here control valves open sequentially tofeed steam to various sectors of the first stage blade circle. However,when a cold turbine is being started, in order to prevent unequalheating and failure of the casing it is customary to open all of thecontrol valves to obtain uniform steam flow and to control the flow by asmall bypass valve built intothe main steam valve from the boiler. Whenit is realized that a modern turbine requires only 1 or 2 percent ofsteam flow to drive an unloaded turbine at full speed the desirabilityof the bypass valve control becomes evident.

So that the turbine may run smoothly with minimum vibration at fullspeed the rotor is designed to have a natural resonant frequency ataround one-third toone-half full speed. The buckets and other componentsmay have resonances over a wide range of speeds. Therefore good practicerequires that as the turbine is brought upto speed it must pass throughthe rotor resonant speed range quickly (no less than one-third themaximum acceleration rate) and if the speed must be held at any pointoff resonance for thermal stress reasons the speed must not stayconstant but must change at a minimum rate (about 3 percent of themaximum acceleration rate) to prevent destructive bucket resonance.

Minor requirements such as maximum acceleration to about one-quarterspeed to permit certain instruments to become operative, and afterloading starts the rapid rise to 3 percent load for boiler stability ora similar purpose are also factors to be considered.

To aid the startup the turbine isusually rotated slowly by a turninggear motor for a number of hours while a little steam flows through toheat the unit uniformly to a temperature compatible with the startingsteam temperature.

A satisfactory steam turbine startup control must take all of the abovelimitations into consideration. There are actually four functions. Firstthe turbine must be accelerated to approximately full speed. Second, theturbine must be synchronized or have its generator output voltage andphase matched to the power system and the circuit closed to it. Third,the turbine output power must be increased to limit of the fully openedbypass valve. And finally, fourth, the control is transferred from thebypass valve to the sequential control valves as the main valve from theboiler is opened wide. Reference may be made to US. Pat. 3,446,224 datedMay 27, 1969 issued to E. E. Zwicky and assigned to the common assigneefor further background infonnation.

The control herein described performs the two most complex functions,acceleration and loading, and monitors the other two for conditionswhich might require preventative manual action.

During acceleration the main gate, described herein, has a maximumoutput of 5 volts DC which, as the input signal to an electrohydraulicor electromechanical servo system, controls the rate of opening of thebypass valve and the turbine acceleration.

As full speed is reached a synchronizing system, not part of thisdisclosure, is actuated to connect the turbine generator to the powersystem. As the main circuit breaker is closed the control-is switched tofunction as a load control It uses the same 5 volt signal as the inputof an auxiliary function described herein which translates it into areversing pulsewidth modulation of constant speed motor which positionsa reference potentiometer. This reference is now the input to thevalveservo mentioned previously which now opens the bypass valve as requiredfor lading control.

After the bypass valve is fully open a transfer action takes place asthe main valve is opened slowly and the sequential control valves areclosed a corresponding amount to provide a smoothly controlled flow ofsteam to the turbine first stage. During the transfer action the controlvalves, steam chest and adjacent areas begin to be subjected to theboiler steam temperature and pressure. Hence the transfer can be limitedto a safe rate by an additional lowest margin input to the main gatedescribed herein although this function is not described in detail inthis disclosure.

2. Statement of Prior Art In the past, it has been necessary in thestartup and initial loading of many large mechanisms (such asturbine-generator sets) to employ extensively trained personnel who werecapable of carefully monitoring each operating stage during startup andrunning of the mechanism (particularly initial loading) usingappropriate and extensive instrumentation. With the continuallyexpanding need for electric power generation equipment, this has becomea troublesome problem with respect to the startup and running ofturbine-generator sets due to the lack of adequately trained personnel,and the expense involved in their training, maintenance of theircompetence, and the installation and maintenance of the extensiveinstrumentation required for safe manual or semiautomatic operation.Turbine-generator sets are typical of many large mechanisms which aremechanically actuated in that they require the opening and closing ofseveral steam supply valves which control admission of steam into theturbine during startup, loading, and running of the set. As pointed outabove, heretofore this has been done manually from infonnative charts.ln addition to these requirements, the startup and initial loading of aturbine-generator set using manual procedures requires a number ofprogressive operating stages in order to assure safe startup and initialloading of the equipment. To overcome certain of these problems, andprovide safer startup, the present invention was developed.

The present invention is intended to provide an electrically operatedcontrol system which can automatically adjust the loading on asteam-turbine generator set to a desired value within an optimum timeperiod with minimum hazard to the equipment involved. To accomplishthis, the new and improved, automatically operating control system ofthe invention has been provided for controlling large, mechanicallyactuated mechanisms. I

lt is therefore a primary object of the present invention to provide anelectrically operable control system for mechanically actuatedmechanisms of large power rating which employ the last adjusted positionof the mechanically actuated mechanism as a memory to provide areference against which further control commands are compared in orderto ascertain the direction and extent of further adjustments to theoperating condition of the mechanism by the control system.

Another object of the invention is to provide a new and improved controlsystem for electrically controlled, mechanically actuated mechanismswherein the mechanism is automatically controlled after startup, duringinitial loading and running of the mechanism in a manner such that anyone of a number of control factors can selectively exercise control overthe operating condition of the mechanism so as to assure safe loadingand running of the equipment within a minimum time period and withminimum hazard to the equipment. This is achieved with the controlsystem of the invention while at the same'time allowing a control factorof primary interest, such as loading, to override and assume control ofthe mechanism upon attaining a predetermined operating condition. I

In practicing the invention a control system for electrically 1controlled, mechanically actuated mechanisms of large power rating (suchas turbine-generator sets) is provided. Such mechanisms generallyinclude mechanical actuating means for. controlling the operatingcondition of the mechanism. For example, in the case of aturbine-generator set the mechanical actuating means would comprise thesteam supply valve between the boiler and the steam turbine; togetherwith its associated bypass, control valves and electrohydraulicactuating mechanisms. The control system according to the inventioncomprises electric power modulation circuit means having its inputcoupled to a, source of electric control signals for controlling theoperation of the mechanism and having its output connected to anelectromechanical positioning means which in turn mechanically controlsthe. mechanically actuated control means of the mechanism. By reason ofthisarrangement, the mechanical actuating means upon being mechanicallypositioned by .the electromechanical positioning means thereafter servesas a memory unit for the control system until the next adjustment of theoperating condition by the source of electric control signals. Theelectric power modulation circuit means preferably comprises a pulsewidth modulation circuit having a source of constant period, pulsedclock signals supplied thereto and two output terminals. The inputcontrol signal serves to selectively control the width of the outputpulses produced by the modulation circuit and selects the outputterminal to which the pulse width modulated output signal pulses aresupplied. The pulse width modulated output signals appearing at one ofthe output terminals then serves to adjust the electromechanicalpositioning means in one direction and the signals from the remainingoutput terminal serve to adjust the positioning means in the otherdirection. The electromechanical positioning means preferably comprisesa servomotor having at least two actuating windings for selectivelydriving the motor in opposite directions and mechanically coupled to andduring the mechanical actuating means of the large mechanism beingcontrolled. The servomotor has one of its actuating windings selectivelyenergized by the pulse width modulated signal derived from one outputterminal of the pulse width modulation circuit for driving the mechanismin a first direction to an extent determined by the pulse widthmodulation of the output signals. The remaining actuating winding of theservomotor is selectively energized by the pulse width modulated outputsignals derived from the remaining.

output terminal for driving the mechanism in a reverse direction.

In a preferred embodiment of the invention, the electric control signalis selected from a plurality of externally derived, electric feedbacksignals representative of a plurality of externally measured quantitiesindicative of a number of operating characteristics of the mechanismbeing controlled. In the case of a turbine-generator set, this pluralityof feedback signals may represent the output indications of sensinginstruments for sensing such factors as turbine casing temperature,turbine speed, expansion, vibration, eccentricity, elongation. Thesesensed signals are compared with corresponding reference or permissiblelevel signals to yield margin signals. These are compared with previousmeasured margins and a predictedmargin value is established. Thesecomputed, predicted future values of certain operating characteristicsare derived from the existing instantaneous measured values by'asuitable logic unit employed in association with the-control system. Theplurality of feedback signals corresponding to stress margins (based ontemperature measurements) are supplied to a main gate selection meanswhich selects the most critical one of these feedback signals for use incontrolling the operation of the mechanism so that at any particularpoint in the operation of the mechanism it is always being controlled inaccordance with its most critical operatin'g condition.

Other objects, features and many of the attendant advantages of thisinvention will be better understood after a reading of the followingdetailed description when considered in connection with the accompanyingdrawings-wherein like parts in each of the several figures arerepresented by the same reference character-e'and wherein:

FIG. 1 is a functional block diagram of anoverall control systemconstructed in accordance with the invention for use in controlling theloading of a steam-turbine generator set;

FIG. 2 is a schematic circuitdiagram of the main gate cir cuit portionof the control system shown in FIG. 1; and

FIG. 3 is a detailedcircuit diagram of the pulse width modulationcircuit and its associated automatic, slow synchronous speed servomotordrive'for use in the present invention.-

. DETAILED DESCRIPTION or PREFERRED EMBODIMENT Overall System I 1 FIG. 1of the drawings is a functional block diagram of. an

overall control system for electrically controlled, mechanicallyactuated mechanisms of large'power rating, constructed in accordancewith the invention. The particular mechanism shown in FIG. 1 comprises aturbine-generator set formed by an electric generator 11 that isdriven'by a high pressure turbine stage 12 and a low pressure turbinestage 13. As is conventional with such turbine-generator sets, the highpressure turbine stage 12 is driven by high pressure, high temperaturesteam from a boiler 14 supplied to the high pressure turbine stage 12through a main steam valve and its associated bypass and control valves,all indicated schematically at 15. The used steam emitted fromthe'discharge end of the high pressure .turbine stage 12 is passed through areheat cycle 16 to the inlet end of the low pressure turbine stage 13and thereafter discharges from the outlet end of the low pressureturbine stage 13 into a condenser 16A where itis condensed into waterand recirculated by a' pump l7 back to the boiler 14. The entirearrangement forms a conventional, closed-cycle steam turbine-generatorset for driving the generator 11 which then generates electric powerthat is supplied to suitable distribution equipment not included as apart of this overall system. It might be mentioned however, that anindication of the power generated by the generator I1 is fed'back to'thecontrol sy stem as a-direct feedback signal for use by the controlsystem in a manner to be described more fully hereinafter.

This feedback signal is obtained over a connection indicated at 18.

The main steam valve and its associated-bypass and control valves,indicated schematically at 15, comprise a mechanical actuating means forthe steam turbine-generator set and are controlled by a suitable valveactuator mechanism shown at 21 that in turn is controlled by anelectrohydraulic control equipment shown at 22. The electrohydrauliccontrol equipment 22 comprises part of an electromechanical positioningmeans for the main steam valve assembly 15 so as to open or close thevalve assembly in accordance with control com mands supplied to theelectrohydraulic control equipment'ZZ. The electrohydraulic controlequipment 22, valve actuator 21- and main steam valve-assembly 15 areall'entirely' conventional in their construction and operation and havebeendescribed in a number of previously published articles and patents.For example, see US. Pat. No. 3,274,443-issued- Sept. 20, i966-forRedundant Speed Control System"--M. A. Eggenberger and P. H.Troutman-lnventors, assigned to the General Electric Company.

The turbine-generator set described above is entirely conventional inits construction and operation and has been described in detail in anumber of previously published articles and patents. Heretofore,however, such turbine-generator sets have been started either manuallyor semiautomatically by skilled personnel who must monitor a number ofpoints in the overall system both as to temperature, pressure,vibration, expansion, predicted temperature changes, etc. so as toassure that during startup, initial loading and running, the equipmentis not damaged. This is particularly critical during the startup andinitial loading of the equipment since at these times the turbinecasing, etc., previously has been cool, and expansion due to thermalchanges, etc., can result in cracking of the turbine casing or othersimilar damage to the equipment.

To avoid damaging the turbine-generator set during initial startup,loading, and running of the equipment, a number of measurements aretaken at various stations located throughout the equipment for measuringtemperature, speed, vibration, expansion, eccentricity, elongation, andother characteristics of the equipment which then can be used incontrolling the equipment or providing necessary alarms so as to assuresafe operation. These measured quantities can be accomplished withconventional measuring devices and indications of the measured valuessupplied to a feedback signal transducer and amplification circuitryindicated at 25. The output from the signal transducer and amplificationcircuit 25 may then be supplied to an alarm. section indicated at 26which then will provide an alarm indicating that at some point in theturbine-generator set the vibration, predicted vibration, differentialexpansion, rotary elongation, eccentricity, etc., is excessive andchanges must be made in the rate of increasing loading, etc., on thegenerator in order to safely achieve operation of the set. The feedbacksignal transducer and amplification circuitry 25 also suppliesa numberof inputs to a computation section or logic unit shown at 27 whichthereafter operates on the feedback signals supplied thereto to providepredicted values of stress margins occurring in the bore and shell ofthe high pressure turbine stage, and reheat bore and shell, and othercritical parts of the equipment where such values can accuratelyindicate safe operation of the turbine-generator set.

The computation section 27 has been described more fully in U.S. Pat.No. 3,340,883-J. R. Peternel and for a more detailed description of thecomputation section 27 reference is made to the above-identified patent.Briefly, however, it can be stated that the computation section 27 issupplied with input information (measurements) relating to thetemperature and speed at particular points on the turbine and reheatstage shells and cores by the feedback signal transducer andamplification section 25, and performs certain calculations using theseinputs to arrive at predicted and actual stress margins for the turbinebore and shell at various critical points in the turbine and reheatstages. Because of the long time constants involved in the arithmeticcalculations being performed by the computation section, thesecalculations are done by digital circuitry. Indications of speed,temperature and other inputs supplied to the computation section aretransformed into digital numbers, and are compared digitally to presetreference values. The differences or safety margins" are thenreconverted to analogue signals which are representative of the actualinstantaneous values of the stress margins and predicted future stressmargins for the turbine bore and shell and reheat stage. The analoguesignals together then with the other actual instantaneous measurementsof margins for vibration, temperature, eccentricity, elongation, etc.,obtained directly from the turbine supervisory instrumentation, aretransmitted both to the alarm section 26, and to a main gate selectionmeans shown at 28 to be described more fully hereinafter.

The main gate selection means 28 is designed to receive the plurality ofanalogue output signals supplied thereto from the computation section 27and to select from this plurality of signals the one analogue signalhaving the lowest value for transmission to the loading control circuitmeans 29. It should be recalled at this point that the computationcircuit 27 accepts temperature and speed data from the feedback signaltransducer and amplification section 25 and performs certaincalculations on this data to arrive at output analogue signalsrepresentative of the temperature and stress margin values, both presentand predicted, for the surface and bore of both the high pressure andreheat stage turbines. It will be appreciated therefore that since thesesignals are representative of both the actual and predicted futurevalues of temperature and stress margins, it is desirable to select fromthese signals that one which has the lowest value for use in controllingthe turbine-generator set in order to assure safe operation of the set.The manner in which the main gate selection means 28 is constructed andoperates to perform this function will be described more fullyhereinafter in connection with FIG. 2 of the drawings.

The loading control circuit 29 is in effect a pulse width modulationcircuit which produces pulse width modulated output electric powersignals that are supplied to the actuating windings of a constant speedservomotor (such as a synchronous or induction motor) shown at 31. Themotor 31 in turn is mechanically coupled through a suitable gearassembly 32 to a reference potentiometer 33 for driving a movablecontact on the reference potentiometer. The reference potentiometer 33in turn is connected to control the operation of the electrohydrauliccontrol equipment 22 that in turn drives the valve actuator mechanism 21that opens and closes the main steam valve assembly and its associatedbypass and control valves indicated at 15. The gear assembly 32 is alsocoupled to a manual reference adjustment knob shown at 34 by means ofwhich the reference potentiometer 33 can be manually adjusted. With thisarrangement the control system of the invention can be overriddenmanually by the manual reference adjustment setting fed into the systemby the knob 34.

The loading control circuit 29 will be described more fully hereinafterin connection with FIG. 3 of the drawings. It

should be noted, however, that the particular embodiment of the controlsystem shown in FIG. 1 is designed for controlling the loading on thegenerator 11. For this reason, a load feedback signal is developed asthe primary control signal for use in the operation of the system, andis supplied back over a direct feedback path 18 to the electrohydrauliccontrol equipment 22 for use by this equipment. This direct feedbackpath is also supplied back through an extension of the conductive path18 shown at 18a to the loading control circuit means 29 for use by thatcircuit in a manner which will be described more fully hereinafter inconnection with FIG. 3.

As mentioned above, the control system illustrated by the block diagramof FIG. 1 is intended for use in controlling the loading of thegenerator 11. For this reason, in the following discussion of theoverall operation of the control system, it will be assumed that thegenerator 11 has been started, and has been brought up to speed so thatnow the problem is initially to load the generator so that it willassume its proportionate share of the electric power (load) to besupplied to some distant facility. Under these assumed conditions, theload feedback path 18 will develop a feedback signal, indicative of theload being supplied by the generator 11, for use by the electrohydrauliccontrol equipment 22 in a known manner. A

portion of the load feedback signal also is supplied back over the path18a for use by the loading control circuit 29 in a manner to bedescribed more fully hereinafter. Further, various instruments locatedat critical points on the turbine equipment for measuring such values astemperature, vibration, expansion, eccentricity, elongation, etc., alsosupply signals indicative of these measured quantities to the feedbacksignal transducer and amplification circuitry shown at 25. Thiscircuitry then supplies certain of the signals directly to the alarmsection 26 for use by that section to indicate to theoperator that anyone of these values has attained an alarm level. In addition, certain ofthe. signals, primarily temperature at specific locations, are suppliedto the computation section 27 which employs these input parameters indeveloping output analogue signals representing the actual present andpredicted future stress margins for the bore and shell of the highpressure and reheat turbine stages.

It has been determined that the predicted stress margin values as wellas the actual value of the temperature within the bore and shell of thehigh pressure and reheat turbine stages are the most critical stressmargins and temperature values occurring in the turbine-generator set.For this reason, the signals representative of these values aredeveloped by the computation section, and supplied through the main gateselection circuit 28 for use by the loading control section 29. Asstated previously the main gate 28 selects the lowest margin signal fromthe computation section for use in controlling the loading control 29.Thereafter, the loading control 29 develops a pulse width modulatedoutput signal that is used in driving the automatic slow synchronousspeed motor- 31 to cause it to adjust the reference potentiometer 33through the reduction gear assembly 32. Adjustment of the referencepotentiometer 33 then controls the operation. of the electrohydrauliccontrol equipment 22 to cause it to move the valve actuator mechanism21. The valve actuator mechanism 21 then in turn opens or closes themain steam control valve and its associated bypass valve and controlvalves indicated at to appropriately adjust the steam supply to the highpressure turbine 12 in accordance with the dictates of the lowest valuecontrol signal supplied to the loading control circuit 29 by the maingate selection circuit means 28.

From a consideration of the above description, it will be appreciatedthat the control system of the invention in effect allows for acontinuous monitoring of the predicted stress margins at several pointson the bore and shell of the high pressure and reheat turbine stages,and thereafter uses the predicted stress margins and actual temperaturevalues at these critical points in controlling loading of the generator11. As a consequence, the loading of the generator to a preset desiredloading level can be achieved in an optimum period of time with minimumhazard to the turbine-generator set. This is accomplished as a result ofthe fact that continuous monitoring of the critical temperatures andspeeds is accomplished along with continuous almost instantaneouscomputation of the actual and predicted stress margins occurring in theturbine casing and bore. Further, it will be appreciated that becausethe lowest value actual or predicted stress margin signal or temperaturesignal operates through the loading control circuit 29 and slowsynchronous motor 31 to set or adjust the valve actuator mechanism 21,and hence the main steam valve and its associated bypass and controlvalve assembly 15, that each such setting of the main steam andassociated bypass and control valve 15 operates in effect to establish areference against which further computed stress margin and temperaturesignals can becompared. Thus, the mechanical positioning of the mainvalve assembly by the actuator 21, electrohydraulic control equipment 22and constant speed motor 31 operates in effect as a memory for theentire system for the purpose of ascertaining whether or not furthercontrol signal inputs from the computation section 27 will effect anincrease or decrease in the loading of the generator 11. The manner inwhich this is accomplished will be described more fully hereinafter inconnection with the detailed description of the loading control circuit29 shown in FIG. 3.

Main Gate Selection Circuit Means The construction of the main gateselection circuit 28 is shown in FIG. 2 of the drawings. As statedpreviously, the main gate selection circuit 28 takes the positive directcurrent analogue signals supplied from the digital to analogue converterin the computation section 27 and determines the one having the lowestmargin value. For this purpose. a plurality of diode gates shown at 41are provided having their cathodes connected to the respective outputsfrom the digital analogue circuits in the computation section 27, andhaving the anodes thereof connected through a common resistor 42 to a+l0 volt supply terminal 43. By reason of this connection, the digitalto analogue signal supplied to that diode gate 41 which has a minimumvalue will allow its diode to conduct, while all the remaining diodeswill be maintained in a blocking condition in the usual manner of adiode gate. The resulting signal developed across the resistor 42 issupplied to the base of a transistor 10 through a Zener diode 44 and themovable contact of a potentiometer 45. The Zener diode 44 comprises apart of a bias circuit which further includes potentiometer 45 thatserves to add a reserve bias of a maximum of 2.5 volts to the zeromargin level signal being applied to the base of transistor 10. As aresult of the addition of this reserve bias, the zero margin level willnot be at a 0 volt level but will have some finite value above 0 voltlevel so as to permit a reserve for overshoot and other similarcontingencies.

In addition to the above connections, the base of the transistor 10 isconnected through a silicon diode 46 to the movable contact of a secondpotentiometer 47. The potentiometer 47 and silicon diode 46 serve tolimit the maximum voltage that canbe applied to the base of thetransistor 10 so as to in effect clip the maximum voltage at apredetermined 5 volt level. The emitter of transistor 10 is connected tothe emitter of a transistor 40 which serves to insert a minimum 1.66voltage level clip into the signal being processed at this point. Thecollector of transistor 40 is connected directly to the 10 volt powersupply terminal .43, and the base of transistor 40 is connected throughthe contact 50 of a critical speed actuated relay to the movable contactof a third potentiometer 48 for setting the minimum 1.66 voltage cliplevel at which the transistor 40 operates. The potentiometers 48 and 47together with the resistors 49, 51 and 52 comprises a voltage dividerfor developing both the 5 volt maximum clip and the [.66 volt minimumclip for the control signal being processed.

The selected signal supplied to the base of transistor 10 by the diodegates 41 is also supplied to the base of a transitor 60 whose emittercollector is connected in series with a resistor 53 across the powersupply terminal 43 and a negative 10 volt power supply terminal 54. Theemitter of transistor 60 is also connected to the base of a transistor50 whose emitter is connected to the ground terminal 55, and whosecollector is connected in series with the actuating winding of anoverstress relay 56. As a consequence of this arrangement, in the eventthat the signals supplied from the diode gate 41 fall below a minimumpredetermined value, the transistor 50 will be rendered conductive andwill actuate the overstress relay 56 to indicate overstress in theturbine casing or core or the existence of a predicted negativetemperature margin.

In the eventuality that the turbine-generator set has not been broughtup to a speed within the critical speed range mentioned previously, thecontacts 50 of the critical speed range relay will not have been closed;The particular control system here described is designed so as to assumean operating condition of a minimum of 10 rpm. at all times except undercircumstances when a hold is called for. Under situations where a holdis called for, the design of the electrohydraulic control 22 is suchthat it takes care of these situations automatically external of thecontrol system of the invention. Similarly, the design is such that itwill automatically handle the turbine during startup prior to reachingthe 10 rpm. assumed minimum acceleration. Thereafter, the control systemshown in FIG. 1 will take over and control operation of theturbine-generator set to assure proper loading at an optimum rate withminimum hazard to the equipment. Because of these circumstances, theoutput from the main gate which is taken off of the terminals 57 and 58,is always limited'to a minimum value of 0.14 volt rather than 0. The0.14 volt corresponds to an acceleration of 10 r.p.m./min. of theturbine-generator set being controlled.

In order to assure that the minimum output voltage of 0.14 volt isalways present at the output terminals .57 and 58 under circumstanceswhere the contacts 50 have not been closed within the critical speedrange, the balanced amplifier comprised by the transistors 20 and 30 isprovided. The balanced amplifier comprised by transistors 20 and 30 isalso required for temperature compensation purposes, and is adjusted bymeans of a set minimum speed potentiometer 59 whose movable contact isconnected to the base of the transistor 30. The minimum voltageappearing at the output terminal 57 is adjusted by adjusting the basepotential of the transistor 30. The minimum voltage level limitingaction takes place through differential action between the transistors2Q and 30 and the diode interconnected between the collector and base ofthe transistor 20 which maintains the outgoing voltage level of terminal57 equal to or above the 0.14 volt minimum level required.

The overall operation of the main gate selection circuit shown in FIG. 2is as follows. If the turbine-generator set is accelerating within thecritical speed range, the contacts 50 will be closed and the outputvoltage appearing at the output terminal 57 will be at a value of I66volts which corresponds to an acceleration of I20 r.p.m./min. asrequired in the critical speed range. If the turbine-generator set isnot operating within this speed range, then the minimum voltage levelappearing at the output terminal 57 will be at the 0.14 volt leveldetermined by the potentiometer 59 and balanced amplifiers 20 and 30. Ithas been previously assumed as a condition of operation; however, thatthe turbine is up to speed and that the loading is to be controlled bythe control system herein described. Accordingly, it will therefore beassumed that the contacts 50 are open and thatcontrolling signals arebeing supplied from the digital to analogue converter circuits of thecomputation section 27 to the diode gates 41.

Under these above-assumed conditions, the diode gate 41 will select theminimum value signal for application to the base of the transistor 10.This minimum signal is added to a reserve bias signal supplied from thepotentiometer 45 and the resultant total signal is clipped or limited toa maximum value of volts by the silicon diode 46 and the setting of thepotentiometer 47. The resultant clipped and augmented signal applied tothe base of transistor thereby controls its conductivity to control theoutput voltage appearing at the output terminal 57 for use incontrolling the operation of the loading control circuit section 29shown in FIG. 1. In the event that this resultant signal tends to gonegative thereby indicating an overstress condition in the turbine, thetransistor 50 will be turned on to actuate the overstress relay 56. Theoutput control voltage appearing at the output terminal 57 is thensupplied to the loading control circuit section 29 shown in FIG. 1 foruse in controlling further operation of the control system.

Loading Control The loading control circuit section 29 of the systemshown in FIG. 1 is illustrated in detail in FIG. 3. The selected controlsignal appearing at the output terminal 57 of the FIG. 2 circuit and tobe applied in the further control of the operation of theturbine-generator set, is supplied to the loading control circuit ofFIG. 3 through the contacts 61 of a relay which is closed at the time ofinitial energization of the system when the main line circuit breaker isclosed. A second pair of contacts 62 is closed concurrently with theclosing of the contact 61 for supplying to the loading control circuit29 square waveshaped clock pulses which are alternate 10 seconds onseconds off square wave pulses varying between a potential level of 6volts positive to 0 volts. These clock pulses are obtained from asuitable clock pulse signal source (not shown) included as part of theoverall system and located in the computation section 27 of the systemarrangement shown in FIG. 1.

The incoming selected control signal supplied from the main gateselection circuit 28 through contact 61 is supplied across a resistor 63to the base of a transistor 50. Transistor SO and transistor form abalanced amplifier arrangement which permits positive and negativecontrol voltages to be developed, and also permits the inclusion of abias adjustment for setting the zero loading level. For this purpose,the collector of the transistor 60 is connected through a conductivepath 64 and a resistor 65 to the base of a transistor that in turncontrols actuation of a raise relay shown at 66 in a manner to bedescribed more fully hereinafter. Similarly. the collector of thetransistor 50 is connected through a conductive path 67 and a resistor65 to the base of a transistor 170 that controls the operation of alower relay 69. Except for certain minor signal shaping operations, theconstant speed motor operation for both the raise and lower function isthe same so that the construction and operation of the circuitry foronly the raise function will be described in detail.

As stated earlier, the loading control circuit 29 has supplied theretoclock signal pulses which alternately vary from a positive 6 volt valueto a 0 volt value at a constant rate. These clock signal pulses aresupplied through the main line breaker closed contacts 62 and resistor71 to the base of a transistor which has its collector connected throughthe resistor 65 to the base of transistor 70. Transistor 70 has itsemitter-collector connected in series with a resistor 72 and a resistor73 across the positive and negative 10 volt power supply terminals 43and 54. The juncture of the'resistor 72 with the collector of transistor70 is connected through a blocking diode 74 to one terminal of a timingcapacitor 75 having the other terminal thereof connected to the 10 voltpositive power supply terminal 43. The first mentioned terminal oftiming capacitor 75 also is connected through a fixed resistor 76, and avariable resistor 77 back to the positive 10 volt power supply terminal43. By this arrangement, variation of the variable resistor 77 cancontrol the RC time constant of the circuit comprised by the timingcapacitor 75 and the two resistors 76 and 77.

The first mentioned terminal of timing capacitor 75 that is connectedthrough blocking diode 74 to the collector of transistor 70 is alsoconnected through a second blocking diode 78 to the base of a transistor90. Transistor 90 has its emitter-collector connected through a resistor79 to a power supply terminal 81 that is supplied through a voltagedropping resistor 82 with the clock pulses applied across contact 62from the clock signal source. The collector of transistor 90 is alsoconnected directly to the base of a transistor 100 and through aresistor 83 to the collector of a PNP power transistor 110. Thisresistor 83 provides a regenerative action which permits the relay to beeither fully on or completely off only. The transistor 100 has itsemitter-collector connected in series with a pair of voltage dividingresistors 84 and 85 across the positive 10 volt power supply terminal 43and the ground terminal 55. The juncture of the voltage dividingresistors 84 and 85 is connected to the base of the PNP transistor 110.The emitter of the transistor 110 is connected to the +6 volt bus.Transistor 110 has its emitter-collector circuit connected in serieswith the actuating windings of the raise relay 66 to a negative 1 8 voltpower source.

In operation, the loading control circuit of FIG. 3 functions in thefollowing manner. During intervals that the signal from the clock sourcesupplied through contact 62 is at its positive 6 volt level, thetransistor 80 is fully conducting. As a result, the base of transistor70 is drawn negative with respect to its emitter, so that transistor 70becomes conducting and the resistor 72 is drawn above ground potential.This results in discharging the timing capacitor 75 above groundpotential so that transistor is allowed to conduct, transistor isrendered nonconducting, and the power transistor operating relay 66likewise is nonconducting. As a consequence, the relay 66 will bedropped out and remains in its nonactuated condition.

. schematically in FIG. 1

During intervals when the clock signal supplied through the contact 62reverts to its zero level value, the transistor 80 is renderednonconducting, and transistor 70 is controlled by the potential suppliedto the base thereof over conductor 64 from the collector of transistor6Q. The conductivity of transistor 60 in turn is controlled by theconduction of transistor 50 that in turn is controlled by the value ofthe control signal supplied through the contacts 61 from the main gateselection circuit. As a consequence, during this interval the timingcapacitor 75 will be charged to some potential below ground level andheld at this point to an extent depending upon the conductivity oftransistor 60 which of course is determined by the value of the controlsignal being supplied over the contacts 61 from the main gate selectioncircuit Concurrently during these same half cycle intervals of the clocksignal, the collector of transistor 90 and the base of transistor 100(which depend upon the clock signal potential as a result of theirconnection to the supply terminal 81) are held at zero potential so thatthese transistors cannot conduct. However,

. when the clock signal changes phase so that the potential of terminal81 is raised to the 6 volt level, transistor 90 can conduct as soon asthe timing capacitor 75 is discharged sufficiently to allow its base tobe brought above ground level. The condenser 86 softens the applicationof voltage to 9Q and 19Q to prevent undesired transients.

The transistor 10, however, can be rendered conductive concurrently withthe change in phase of the clock signal, and

in turn causes the power transistor 110 to be rendered conductive andactivate the raise relay 66. Concurrently with this action, the rise ofthe clock potential has started transistor 80 conducting so thattransistor 70 is then rendered fully conducting, and the timingcapacitor 75 starts to discharge ina positive direction until transistor90 turns on. Upon turn-on of 90, the transistors 100 and 110 are turnedoff thereby determining the interval of time that the raise relay 66 isactivated. The rate at which the timing capacitor 75 discharges isdetermined by the setting of the variable resistor 77, and the length oftime that is required to turn on transistor 90 is of course determinedby the level to which it was charged during the preceding voltage levelhalf cycle of the clock signal. This length of time determines the pointof turn-off transistors 100 and 110 and hence determines the pulse widthmodulation of the signal pulse applied to the actuating winding of theraise relay 66. If during a preceding 6 volt level half cycle of theclock signal, timing capacitor 75 has not been charged below groundlevel by conduction of transistors 60 and 70, then 90 turns onconcurrently with the changein phase of the clock signal, andtransistors 100 and llQare maintained off. Under these circumstancesraise relay 66 will not be activated.

Operation of the circuit with respect to the actuation of the lowerrelay 69 is similar to that described above with respect to the raiserelay 66. For this reason, corresponding elements of the lower relaycircuit have been identified with the same reference character primedand the'transistors employed have been raised in identifying referencenumeral by a factor of IO. For example, the transistor corresponding totransistor 70 is identified as 170, that corresponding to 80 is 180, andso on.

I In all other respects, the operation of the lower relay circuit is thesame as that of the raise relay.

FIG. 3a of the drawings illustrates the wiring diagram for the slowsynchronous speed servomotorwhich is energized by the raise and lowerrelays 66 and 69, respectively. The contacts of the raise relay 66 areshown at 66' and the contacts of the lower relay 69 are shown at 69'. Itwill be appreciated that in FIG. 30 only the actuating windings 91 and92 of the servomotor are illustrated. The rotor of the motor would ofcourse be mechanically coupled to the reducing gear assembly shown thatdrives the electrohydraulic valve control mechanism of the system. Theactuating winding 91 will be connected across a power supply source foractuating the servomotor which in the embodiment of the inventiondisclosed comprises a l volt, 60 cycle alternating current source.Excitation of the raise winding 91 will occur when the contacts 66 areclosed during excitation of the raise relay 66 by the loading controlcircuit of FIG. 3. Similarly, excitation of the lower winding 92 willoccur during closure of the contacts 69 by the loading control circuit.Protective thyristors shown at 93 and 94 are connected across each ofthe windings 91 and 92, and phase-splitting capacitor 95 and a currentlimiting resistor 96 are connected across both windings in aconventional manner.

If desired, it is entirely possible to replace the excitation circuit ofFIG. 3a with the power semiconductor circuit configuration shown in FIG.3b. With the excitation circuit of FIG. 3b, a power ratedsemiconductortriode known commercially as the triac indicated at 97 acan be inserted in place of one of the contacts such as 66' in theseries with the raise actuating winding 91. A similar power triac (notshown) would be connected in series with the lower winding 92 in placeof the relay contact 69'. With such an arrangement, turn-on and'turn-offof the triac 97 would be controlled directly by the transistor,

such as 110, in the pulse width modulation control circuit.

Under such circumstances, transistor 11 would have its emitter-collectorconnected across a pair of voltage dividing resistors 98 and 99 which inturn are connected to the control gate of the triac 97. Upon turn-on ofthe transistor 110, a turn-on gating pulse will be applied to the triac97 which then in turn would be turned on to excite the actuating winding91. A similar connection would be required for the triac exciting thelower actuating winding 92 inorder to drive the servomotor in thereverse or lower direction.

Referring back to FIG. 1 of the drawings, it should be remembered thatin addition to the externally derived feedback signals which areindicative of the actual instantaneous stress margins and predictedstress margins as well as temperatures in the turbine case and bore,supplied through the main gate 28 to the loading control circuit 29, adirect feedback signal of primary interest issupplied thereto over thepath 180. This direct feedback signal is in effect a signalrepresentative of the loading on thegenerator 11. As shown in FIG. 3 ofthe drawings, this direct feedback signal supplied over conductor path18a is applied across a pair of voltage dividing resistors 101 and 102which are also excited from the movable contact of a terminal load setreference potentiometer 103. The parameters of the circuit are adjustedso that a -l0 volt load feedback signal is representative of I00 percentload on the turbine-generator set. Since the terminal load set referencepotentiometer 103 applies its'output voltage across the voltage dividingresistors 101 and 102 in opposition to the load feedback signal suppliedto the circuit across path 18a, the maximum of the terminal load setpotentiometer 103 is adjusted to equal the maximum load feedback signalof IO volts. The juncture of the voltage dividing resistors 101 and 102is connected to the base of a transistor 10 which is connected in abalanced amplifier circuit arrangement with a second transistor 20 whosecollector is connected directly to the collector of transistor 60. Bythis arrangement, if at any point during the loading of theturbine-generator set, the load feedback signal supplied over the path18a approaches the value of the terminal load fed into the terminal loadset potentiometer 103, the transistor 10 will gradually turn off. Upontransistor 10 being burned off, transistor 20 turns on and loads downthe collector of transistor 60 so as to prevent it from permitting araise signal to be supplied to the raise relay 66 in the previouslydescribed manner. A potentiometer 104 has its movable contact connectedto the base of the transistor 20 to provide a set balance adjustment forthe transistor 20 in initially aligning the circuit for operation.

In a similar manner, a pair of balanced transistors 30 and 40 willoperate to maintain a minimum loading setting as adjusted by a minimumload reference potentiometer shown at 106 in the event that the loadfeedback signal across conductor 18a drops to a value less than therequired 0.3 volt representing 3 percent loading on theturbine-generator set. It will be seen that in the event that thefeedback signal drops below this minimum set value, the transistor 30 isswitched off, and the transistor TQ rendered conductive. Upon transistor40 being rendered conductive, a turn-on potential is applied through theblocking diode 107 to the base of transistor 50 to cause this transistorto actuate the raise relay 66 in the previously described manner.

In addition to the above adjustments, there is a further requirementthat means be provided for adjusting the zero level condition. By thezero level loading condition is meant that position of the main steamvalveand its associated bypass and control valves, which is consideredto be in neutral, will neither open or close further due to the value ofthe signals (known as the zero loading level value) supplied to theloading control circuitry. For this purpose, a zero loading adjustingpotentiometer 108 is provided for adjusting the bias applied to the baseof the transistor 50 and an input balance potentiometer 109 is providedfor adjusting the bias applied to the base of the transistor 60. Thewidth of this zero loading level value can be adjusted by apotentiometer 111 connected to the collector of transistor 60. Withthese potentiometers, the zero level adjustment can be made and isnormally adjusted to be within the range of to 2%volts positive. if thezero level is set at the Zhvolt value, there will be an equal range fora raising and lowering of the control system. Obviously, however, ifthis value is set at the 0 volt level it would leave no range forlowering of the loading condition by closing of the main steam valve. Itis assumed, however, that in the normal operation of the circuit, thezero level adjustment will be set at the 2%volt value so as to providefor both further opening and further closing of the steam valve by thecontrol system.

In the operation of the loading control circuit shown in H6. 3, theterminal and minimum load set are first adjusted by appropriate settingof the potentiometers 103 and 106. Zero loading level is then adjustedby appropriate setting of the potentiometers 108, 109 and 111 to providefor both raising and lowering of the servodrive motor. The directcurrent analogue signal from the main gate calling for either raising orlowering of the steam valve is supplied through the closed contacts 61to the balanced amplifier comprised by transistors 50 and 6Q. Balancedamplifier Q, 60 then functions to develop pulse width modulatedcontrolling signals that are applied to the actuating windings of theservomotor controlling the opening or closing of the steam valve of theturbine-generator set to thereby properly adjust the loading condition.In the event that the loading drops below a minimum 3 percent value,transistor 40 is rendered conductive and the control circuitautomatically increases the loading on the set. Upon reaching theterminal load set into the potentiometer 103, transistor 2Q conducts andloads down the collector of transistor 60 to prevent further raising oropening of the main steam valve. ln the intervening periods of operationseveral of the DC analogue signals being supplied to contact 61 from themain selection circuit adjust the opening of the main steam valve up ordown in a manner depending upon the value of the analogue signal appliedthereto from the computation section 27 through the main gate selectedselection circuit. During this adjustment period that last set positionof the valve assembly serves as a mechanical memory for the system.

From the foregoing description it will be appreciated that the inventionmakes available a new and improved control system for electricallycontrolled, mechanically actuated mechanisms of large power rating suchas turbine-generator sets. While the specific embodiment of theinvention disclosed is intended for use with a turbine-generator set, itis believed obvious that the system can be applied equally well to othersimilar mechanically actuated mechanisms such as automaticallycontrolled machine tools, milling machines, lathes, drill presses andthe like, wherein the mechanically actuated mechanism upon beingpositioned can serve as a memory for further operation of the controlsystem. The control system can be used with any apparatus wherein it isdesired to automatically control startup, initial loading and/or runningof the mechanism in a manner such that a number of control factors canselectively exercise control over the operating condition of themechanism thereby assuring safe startup and loading within an optimumtime period with minimum hazard to the equipment. This is achieved whileallowing a control factor of primary interest such as loading, speed,position, etc., to override and assume control of the mechanism uponobtaining a predetermined operating condition.

Having described one specific embodiment of a control system constructedaccording to the invention, it is believed obvious that othermodifications and variations of the invention will be suggested to thoseskilled in the art in the light of the above teachings. lt is thereforeto be understood that changes may be made in the particular embodimentof the invention described, and that any such changes that come withinthe spirit and scope of the invention as defined by the appended claims,are intended to be covered.

lclaim:

1. A control system for electrically controlled, mechanically actuatedpower mechanisms having mechanical actuating means for controlling theoperating condition of the mechanism, means for measuring a plurality ofstress margins developed during operation of said mechanisms, means forselecting the predominant one of said measured margins to provide asource of electric control signals, the control system comprisingelectric power modulation circuit means having its input coupled to saidsource of electric control signals for controlling the operation of themechanism and for producing pulses of alternating electric power havinga pulse width which is varied in accordance with an operatingcharacteristic of said electric control signals, and an AC motor havingits output mechanically coupled to and controlling the position of saidmechanical actuating means and having its input coupled to andcontrolled by the output from the electric power modulation circuitmeans, said mechanical actuating means being mechanically positioned bythe electromechanical positioning means and upon being so positionedthereafter serves as a memory unit forthe control system until the nextadjustment of the operating condition by the source of electric controlsignals.

2. A control system according to claim 1 wherein said electric powermodulation circuit means comprises a pulse width modulation circuithaving a source of constant frequency AC power supplied thereto and twooutput terminals, the input control signal serving to selectivelycontrol the width of the pulses of electric power produced by themodulation circuit and select the output terminal to which such pulsesof electric power are supplied, the pulses of electric power from one ofthe output terminals serving to drive the AC motor in one direction andthe pulses of electric power from the remaining terminal serving todrive said motor in the other direction, and an AC power output stageselectively excited by the pulses of electric power for driving the ACmotor.

3. A control system according to claim 2 wherein the AC motor comprisesa servomotor having at least two actuating windings for selectivelydriving .the motor in opposite directions and being mechanically coupledto and driving the mechanical actuating means, the said servomotorhaving one of its actuating windings selectively energized by the seriesof pulses of electric power derived from one output terminal for drivingthe AC motor'in a first direction to an extent determined by theelectric control signals, and having the remaining actuating windingselectively energized by the series of pulses of electric power derivedfrom the remaining output terminal for driving the AC motor in thereverse direction to an extent determined by the electric controlsignals.

4. A control system according to claim 3 wherein the power output stagecomprises a pair of relays having the exciting windings thereofconnected to respective ones of the output terminals for excitation bythe pulses of electric power and having the contacts thereof connectedto appropriate actuating windings of the servomotor motor along with asource of excitation power to control operation of the servomotor.

5. A control system according to claim 3 wherein the power output stagecomprises a pair of power semiconductor devices having the controlelements thereof connected to respective ones of the output terminalsfor excitation by the'pulses of electric power and having the loadterminals thereof connected to appropriate actuating windings of theservomotor along with a source of excitation power to control operationof the servomotor.

6. A control system for electrically controlled, mechanically actuatedmechanisms of large power rating having mechanical actuating means forcontrolling the operating condition of a mechanism, said control systemcomprising means for developing a plurality of externally derivedelectric feedback signals representative of a plurality of externallymeasured quantities indicative of a number of stress margincharacteristics of the mechanism being controlled, main gate selectionmeans having the plurality of externally derived feedback signalssupplied thereto as inputs for selecting a desired one of the externallyderived feedback signals for use in controlling operation of themechanism, electric power modulation circuit means having its inputcoupled to the output from the main gate selection means for producingpulses of AC electric power, and an AC motor, means for adjusting thespeed of said AC motor comprising means for applying pulses of ACelectric power from said said electric'power modulation circuit means tothe input of said AC motor and means for varying the width of saidpulses of AC electric power in accbrdance with the output from themaingate selection means, said mechanical actuating means being mechanicallypositioned by the AC motor and upon being so positioned thereafterserves as a memory unit for the control system until the next adjustmentof the operating condition of the mechanism by the control signalsupplied by the main gate selection means.

7. A control system according to claim 6 wherein said electric powermodulation circuit means comprises a pulse width modulation circuithaving a source of constant period, pulsed AC electric power suppliedthereto and two output terminals, the input control signal serving toselectively control the width of the pulses of AC electric powerproduced by the modulation circuit and select the output terminal towhich the pulses of AC electric power are supplied, pulses of ACelectric power from one of the output terminals serving to adjust theelectromechanical positioning means in one direction and the pulses ofAC electric power from the remaining terminal serving to adjust thepositioning means in the other direction, and a power output stageselectively excited by the pulses of AC electric power for driving theelectromechanical positioning means.

8. A control system according to claim 7 further including circuit meansfor adjusting the zero bias level of the pulse width modulation circuitso that it responds in a different manner to incoming control signalsfrom the main gate selection means which are over or under this zerobias level to thereby derive the pulses of AC electric power for each ofthe output terminals thereof, the pulses of AC electric power beingapplied to one output terminal in response to the incoming controlsignal exceeding thezero bias level and being applied to the otheroutput terminal in response to the incoming control signal droppingbelow the zero bias level.

9. A control system for electrically controlled, mechanically actuatedmechanisms of large power rating having mechanical actuating means forcontrolling the operating condition of a mechanism, said control systemcomprising means for developing a plurality of externally derivedelectric feedback signals representative of a number of stress margincharacteristics of the mechanism being controlled, main gate selectionmeans having the plurality of externally derived feedback signalssupplied thereto as inputs for selecting a desired one of the externallyderived feedback signals for use in controlling operation of themechanism, electric power modulation circuit means having its inputcoupled to the output from the main gate selection means for modulatedcontrol electric power signals, electromechanical positioning meanshaving its output mechanically coupled to and controlling the positionof said mechanical actuating means and having its input coupled to andcontrolled by the output from the electric power modulation circuitmeans, said mechanical actuating means being mechanically positioned bythe electromechanical positioning means and upon being so positionedthereafter serves as a memory unit for the control system until the nextadjustment of the operating condition of the mechanism by the controlsignal supplied by the main gate selection means, direct feedback meansfor deriving a primary feedback signal that is directly related to anoperating condition of the controlled mechanism that is of primaryinterest in the control of the mechanism, said direct feedback meansbeing coupled to said electric power modulation circuit means as anadditional input thereto, and override control circuit means comprisinga part of said electric power modulation circuit means and controlled bysaid primary feedback signal for overriding and taking over control ofthe operation of said electric power modulation circuit means inresponse to the primary feedback signal attaining a predetermined value.

10. A control system according to claim 9 wherein the override controlcircuit means comprises a terminal load set potentiometer connected inopposition to the direct feedback connection to the pulse widthmodulation circuit whereby upon the primary feedback signal exceedingthe terminal load value set into the potentiometer, further productionof pulse width modulated output signals in the increasing load directionis prevented.

11. A control system according to claim 10 wherein the override controlcircuit meansfurther includes a minimum load potentiometer andcomparison circuit means for comparing the primary feedback signal tothe minimum load value set on the minimum load potentiometer and foroverriding control of the pulse width modulation circuit upon theprimary feedback signal dropping below the preset minimum load value tothereby cause the pulse width modulation circuit to produce output pulsewidth modulated signals at the output terminal thereof which will causethe load to increase.

12. A control system according to claim 11 wherein the mechanism beingcontrolled comprises a steam turbinegenerator set, the mechanicalactuating means comprises the main steam supply valve from the boiler tothe turbine and its associated bypass and control valves andelectrohydraulic actuating mechanism, the means for developing aplurality of externally derived electric feedback signals comprises aplurality of sensing instruments for sensing turbine casing temperature,pressure, turbine speed, expansion, vibration, eccentricity, elongation,etc., together with computation circuits which derive predicted futurevalues of such operating conditions from the existing instantaneousmeasured values, and the electromechanical positioning means comprises aconstant speed servomotor mechanically driving a reference potentiometerthat in turn controls the electrohydraulic valve actuating mechanism ofthe steam turbine, the said servomotor having one of its actuatingwindings selectively energized by the series of pulse width modulatedcontrol signals derived from one of the output terminals of the pulsewidth modulation circuit to cause the servomotor to drive the referencepotentiometer in a direction to increase loading on theturbine-generator and having the other of its actuating windingsselectively energized by the series of pulse width modulated controlsignals derived from the other output terminal of the pulse widthmodulation circuit to cause the servomotor to drive the referencepotentiometer in a direction to decrease loading on the turbinegeneratorset.

13. A control system according to claim 12 further characterized in thatthe direct feedback means comprises a loadv feedback signal from thegenerator to the pulse width modulation circuit and constitutes primaryfeedback signal that is directly related to the loading on theturbine-generator set.

14. A control system according to claim 13 wherein the mechanicalconnection of the servomotor to the reference potentiometer is through asuitable gear assembly that is also geared to and selectively driven bya manual reference adjustment knob for manually overriding andadditionally controlling the operation of the turbine-generator set.

15. A control system according to claim 14 wherein the power outputstage of the pulse width modulation circuit comprises a pair of relayshaving the exciting windings thereof connected to respective ones of theoutput terminals of the pulse width modulation circuit for excitation bythe pulse width modulated clock pulses and having the contacts thereofconnected through a source of energizing power to appropriate actuatingwindings of the servomotor to control the direction and extent ofmovement of the servomotor.

16. A control system according to claim 14 wherein the power outputstage of the pulse width modulation circuit comprises a pair of powersemiconductor devices having the control elements thereof connected torespective ones of the output terminals of the pulse width modulationcircuit for excitation by the pulse width modulated clock pulses andhaving the load terminals thereof connected to appropriate actuatingwindings of the servomotor to control the direction and extent ofmovement of the servomotor.

17. In an operating system which produces differently measurablequantities representing the operating conditions of said system, meansfor controlling said operating system comprising means for providingfirst signals representative of stress margins developed by saidoperating system, means for selecting and processing desired ones ofsaid first signals to produce a plurality of different control signals,said means for selecting and processing desired ones of said controlsignals comprising a source of reference signals and means for comparinggiven ones of said first signals with corresponding ones of saidreference signals to produce said control signals, a source of clocksignals having a given recurrence rate, means for producing outputsignals, means responsive to said given recurrence rate clock signals tocontrol the recurrence rate of said output signals, means responsive toselected ones of said control signals to control the number of timerecurrences of said output signals at said recurrence rate, a motor, andmeans for controlling motor action comprising means for applying saidoutput signals to said motor.

18. An arrangement according to claim 17 further comprising a source ofadditional control signals, and means responsive to said additionalcontrol signals and said motor action to produce a resultant outputcontrol signal, and means for controlling said operating system withsaid resultant output control signal.

19. An arrangement according to claim 17 further comprising a signaldivider for producing resultant signal s signals having a signalcharacteristic adjustable between a maximum and a minimum value, andmeans for controlling the value of said signal characteristic inresponse to said motor action.

20. A turbine loading control closed cycle system comprising a constantspeed servomotor, a source of clock pulses, a source of first controlsignals representing stress margins developed by said turbine duringoperation, means responsive to said clock pulses for providing pulsed ACpower of given frequency for controlling motor operation, means forcontrolling the duration of said power pulses of given frequency as afunction of said first signals, and means responsive to said motoroperation to provide control without overcontrol or hunting.

21. A turbine loading control comprising a source of analogue signalsrepresenting a plurality of stress margins developed during operation ofsaid turbine, means for selecting the most critical of said signals andconverting said selected signals to pulses of AC power of givenfrequency whose pulse width varies as a function of the magnitude ofsaid most critical of said signals, a constant speed AC servomotor, acontrollable system, means for adjusting the average speed of said ACmotor comprising said motor responsive to said pulses of electric powerto provide proportional control of said system.

22. An arrangement according to claim 21 further comprising means forconstrainingmotor speed between given values system output between givenvalues and the balance level between directions of rotation of saidservomotor.

1. A control system for electrically controlled, mechanically actuatedpower mechanisms having mechanical actuating means for controlling theoperating condition of the mechanism, means for measuring a plurality ofstress margins developed during operation of said mechanisms, means forselecting the predominant one of said measured margins to provide asource of electric control signals, the control system comprisingelectric power modulation circuit means having its input coupled to saidsource of electric control signals for controlling the operation of themechanism and for producing pulses of alternating electric power havinga pulse width which is varied in accordance with an operatingcharacteristic of said electric control signals, and an AC motor havingits output mechanically coupled to and controlling the position of saidmechanical actuating means and having its input coupled to andcontrolled by the output from the electric power modulation circuitmeans, said mechanical actuating means being mechanically positioned bythe electromechanical positioning means and upon being so positionedthereafter serves as a memory unit for the control system until the nextadjustment of the operating condition by the source of electric controlsignals.
 2. A control system according to claim 1 wherein said electricpower modulation circuit means comprises a pulse width modulationcircuit having a source of constant frequency AC power supplied theretoand two output terminals, the input control signal serving toselectively control the width of the pulses of electric power producedby the modulation circuit and select the output terminal to which suchpulses of electric power are supplied, the pulses of electric power fromone of the output terminals serving to drive the AC motor in onedirection and the pulses of electric power from the remaining terminalserving to drive said motor in the other direction, and an AC poweroutput stage selectively excited by the pulses of electric power fordriving the AC motor.
 3. A control system according to claim 2 whereinthe AC motor comprises a servomotor having at least two actuatingwindings for selectively driving the motor in opposite directions andbeing mechanically coupled to and driving the mechanical actuatingmeans, the said servomotor having one of its actuating windingsselectively energized by the series of pulses of electric power derivedfrom one output terminal for driving the AC motor in a first directionto an extent determined by the electric control signals, and having theremaining actuating winding selectively energized by the series ofpulses of electric power derived from the remaining output terminal fordriving the AC motor in the reverse direction to an extent determined bythe electric control signals.
 4. A control system according to claim 3wherein the power output stage comprises a pair of relays having theexciting windings thereof connected to respective ones of the outputterminals for excitation by the pulses of electric power and having thecontacts thereof connected to appropriate actuating windings of theservomotor motor along with a source of excitation power to controloperation of the servomotor.
 5. A control system according to claim 3wherein the power output stage comprises a pair of power semiconductordevices having the control elements thereof connected to respective onesof the output terminals for excitation by the pulses of electric powerand having the load terminals thereof connected to appropriate actuatingwindings of the servomotor along with a source of excitation power tocontrol operation of the servomotor.
 6. A control system forelectrically controlled, mechanically actuated mechanisms of large powerrating having mechanical actuating means for controlling the Operatingcondition of a mechanism, said control system comprising means fordeveloping a plurality of externally derived electric feedback signalsrepresentative of a plurality of externally measured quantitiesindicative of a number of stress margin characteristics of the mechanismbeing controlled, main gate selection means having the plurality ofexternally derived feedback signals supplied thereto as inputs forselecting a desired one of the externally derived feedback signals foruse in controlling operation of the mechanism, electric power modulationcircuit means having its input coupled to the output from the main gateselection means for producing pulses of AC electric power, and an ACmotor, means for adjusting the speed of said AC motor comprising meansfor applying pulses of AC electric power from said said electric powermodulation circuit means to the input of said AC motor and means forvarying the width of said pulses of AC electric power in accordance withthe output from the main gate selection means, said mechanical actuatingmeans being mechanically positioned by the AC motor and upon being sopositioned thereafter serves as a memory unit for the control systemuntil the next adjustment of the operating condition of the mechanism bythe control signal supplied by the main gate selection means.
 7. Acontrol system according to claim 6 wherein said electric powermodulation circuit means comprises a pulse width modulation circuithaving a source of constant period, pulsed AC electric power suppliedthereto and two output terminals, the input control signal serving toselectively control the width of the pulses of AC electric powerproduced by the modulation circuit and select the output terminal towhich the pulses of AC electric power are supplied, pulses of ACelectric power from one of the output terminals serving to adjust theelectromechanical positioning means in one direction and the pulses ofAC electric power from the remaining terminal serving to adjust thepositioning means in the other direction, and a power output stageselectively excited by the pulses of AC electric power for driving theelectromechanical positioning means.
 8. A control system according toclaim 7 further including circuit means for adjusting the zero biaslevel of the pulse width modulation circuit so that it responds in adifferent manner to incoming control signals from the main gateselection means which are over or under this zero bias level to therebyderive the pulses of AC electric power for each of the output terminalsthereof, the pulses of AC electric power being applied to one outputterminal in response to the incoming control signal exceeding the zerobias level and being applied to the other output terminal in response tothe incoming control signal dropping below the zero bias level.
 9. Acontrol system for electrically controlled, mechanically actuatedmechanisms of large power rating having mechanical actuating means forcontrolling the operating condition of a mechanism, said control systemcomprising means for developing a plurality of externally derivedelectric feedback signals representative of a number of stress margincharacteristics of the mechanism being controlled, main gate selectionmeans having the plurality of externally derived feedback signalssupplied thereto as inputs for selecting a desired one of the externallyderived feedback signals for use in controlling operation of themechanism, electric power modulation circuit means having its inputcoupled to the output from the main gate selection means for modulatedcontrol electric power signals, electromechanical positioning meanshaving its output mechanically coupled to and controlling the positionof said mechanical actuating means and having its input coupled to andcontrolled by the output from the electric power modulation circuitmeans, said mechanical actuating means being mechanically positioned bythe electromechanical positioning means and uPon being so positionedthereafter serves as a memory unit for the control system until the nextadjustment of the operating condition of the mechanism by the controlsignal supplied by the main gate selection means, direct feedback meansfor deriving a primary feedback signal that is directly related to anoperating condition of the controlled mechanism that is of primaryinterest in the control of the mechanism, said direct feedback meansbeing coupled to said electric power modulation circuit means as anadditional input thereto, and override control circuit means comprisinga part of said electric power modulation circuit means and controlled bysaid primary feedback signal for overriding and taking over control ofthe operation of said electric power modulation circuit means inresponse to the primary feedback signal attaining a predetermined value.10. A control system according to claim 9 wherein the override controlcircuit means comprises a terminal load set potentiometer connected inopposition to the direct feedback connection to the pulse widthmodulation circuit whereby upon the primary feedback signal exceedingthe terminal load value set into the potentiometer, further productionof pulse width modulated output signals in the increasing load directionis prevented.
 11. A control system according to claim 10 wherein theoverride control circuit means further includes a minimum loadpotentiometer and comparison circuit means for comparing the primaryfeedback signal to the minimum load value set on the minimum loadpotentiometer and for overriding control of the pulse width modulationcircuit upon the primary feedback signal dropping below the presetminimum load value to thereby cause the pulse width modulation circuitto produce output pulse width modulated signals at the output terminalthereof which will cause the load to increase.
 12. A control systemaccording to claim 11 wherein the mechanism being controlled comprises asteam turbine-generator set, the mechanical actuating means comprisesthe main steam supply valve from the boiler to the turbine and itsassociated bypass and control valves and electrohydraulic actuatingmechanism, the means for developing a plurality of externally derivedelectric feedback signals comprises a plurality of sensing instrumentsfor sensing turbine casing temperature, pressure, turbine speed,expansion, vibration, eccentricity, elongation, etc., together withcomputation circuits which derive predicted future values of suchoperating conditions from the existing instantaneous measured values,and the electromechanical positioning means comprises a constant speedservomotor mechanically driving a reference potentiometer that in turncontrols the electrohydraulic valve actuating mechanism of the steamturbine, the said servomotor having one of its actuating windingsselectively energized by the series of pulse width modulated controlsignals derived from one of the output terminals of the pulse widthmodulation circuit to cause the servomotor to drive the referencepotentiometer in a direction to increase loading on theturbine-generator and having the other of its actuating windingsselectively energized by the series of pulse width modulated controlsignals derived from the other output terminal of the pulse widthmodulation circuit to cause the servomotor to drive the referencepotentiometer in a direction to decrease loading on theturbine-generator set.
 13. A control system according to claim 12further characterized in that the direct feedback means comprises a loadfeedback signal from the generator to the pulse width modulation circuitand constitutes primary feedback signal that is directly related to theloading on the turbine-generator set.
 14. A control system according toclaim 13 wherein the mechanical connection of the servomotor to thereference potentiometer is through a suitable gear assembly that is alsogeared to and selectively driven by a manual reference adjustment knobfor manually overriding aNd additionally controlling the operation ofthe turbine-generator set.
 15. A control system according to claim 14wherein the power output stage of the pulse width modulation circuitcomprises a pair of relays having the exciting windings thereofconnected to respective ones of the output terminals of the pulse widthmodulation circuit for excitation by the pulse width modulated clockpulses and having the contacts thereof connected through a source ofenergizing power to appropriate actuating windings of the servomotor tocontrol the direction and extent of movement of the servomotor.
 16. Acontrol system according to claim 14 wherein the power output stage ofthe pulse width modulation circuit comprises a pair of powersemiconductor devices having the control elements thereof connected torespective ones of the output terminals of the pulse width modulationcircuit for excitation by the pulse width modulated clock pulses andhaving the load terminals thereof connected to appropriate actuatingwindings of the servomotor to control the direction and extent ofmovement of the servomotor.
 17. In an operating system which producesdifferently measurable quantities representing the operating conditionsof said system, means for controlling said operating system comprisingmeans for providing first signals representative of stress marginsdeveloped by said operating system, means for selecting and processingdesired ones of said first signals to produce a plurality of differentcontrol signals, said means for selecting and processing desired ones ofsaid control signals comprising a source of reference signals and meansfor comparing given ones of said first signals with corresponding onesof said reference signals to produce said control signals, a source ofclock signals having a given recurrence rate, means for producing outputsignals, means responsive to said given recurrence rate clock signals tocontrol the recurrence rate of said output signals, means responsive toselected ones of said control signals to control the number of timerecurrences of said output signals at said recurrence rate, a motor, andmeans for controlling motor action comprising means for applying saidoutput signals to said motor.
 18. An arrangement according to claim 17further comprising a source of additional control signals, and meansresponsive to said additional control signals and said motor action toproduce a resultant output control signal, and means for controllingsaid operating system with said resultant output control signal.
 19. Anarrangement according to claim 17 further comprising a signal dividerfor producing resultant signal s signals having a signal characteristicadjustable between a maximum and a minimum value, and means forcontrolling the value of said signal characteristic in response to saidmotor action.
 20. A turbine loading control closed cycle systemcomprising a constant speed servomotor, a source of clock pulses, asource of first control signals representing stress margins developed bysaid turbine during operation, means responsive to said clock pulses forproviding pulsed AC power of given frequency for controlling motoroperation, means for controlling the duration of said power pulses ofgiven frequency as a function of said first signals, and meansresponsive to said motor operation to provide control withoutovercontrol or hunting.
 21. A turbine loading control comprising asource of analogue signals representing a plurality of stress marginsdeveloped during operation of said turbine, means for selecting the mostcritical of said signals and converting said selected signals to pulsesof AC power of given frequency whose pulse width varies as a function ofthe magnitude of said most critical of said signals, a constant speed ACservomotor, a controllable system, means for adjusting the average speedof said AC motor comprising said motor responsive to said pulses ofelectric power to provide proportional control of said system.
 22. Anarrangement according to claim 21 further comprising means forconstraining motor speed between given values, system output betweengiven values and the balance level between directions of rotation ofsaid servomotor.